Pretherapeutic screening for Dihydropyrimidine deshydrogenase deficiency in measuring uracilemia in dialysis patients leads to a high rate of falsely positive results

Impact of renal impairment on dihydropyrimidine dehydrogenase (DPD) phenotyping

BACKGROUND

The chemotherapeutic agent 5-fluorouracil (5-FU) is catabolized by dihydropyrimidine dehydrogenase (DPD), the deficiency of which may lead to severe toxicity or death. Since 2019, DPD deficiency testing, based on uracilemia, is mandatory in France and recommended in Europe before initiating fluoropyrimidine-based regimens. However, it has been recently shown that renal impairment may impact uracil concentration and thus DPD phenotyping.

PATIENTS AND METHODS

The impact of renal function on uracilemia and DPD phenotype was studied on 3039 samples obtained from three French centers. We also explored the influence of dialysis and measured glomerular filtration rate (mGFR) on both parameters. Finally, using patients as their own controls, we assessed as to what extent modifications in renal function impacted uracilemia and DPD phenotyping.

RESULTS

We observed that uracilemia and DPD-deficient phenotypes increased concomitantly to the severity of renal impairment based on the estimated GFR, independently and more critically than hepatic function. This observation was confirmed with the mGFR. The risk of being classified 'DPD deficient' based on uracilemia was statistically higher in patients with renal impairment or dialyzed if uracilemia was measured before dialysis but not after. Indeed, the rate of DPD deficiency decreased from 86.4% before dialysis to 13.7% after. Moreover, for patients with transient renal impairment, the rate of DPD deficiency dropped dramatically from 83.3% to 16.7% when patients restored their renal function, especially in patients with an uracilemia close to 16 ng/ml.

CONCLUSIONS

DPD deficiency testing using uracilemia could be misleading in patients with renal impairment. When possible, uracilemia should be reassessed in case of transient renal impairment. For patients under dialysis, testing of DPD deficiency should be carried out on samples taken after dialysis. Hence, 5-FU therapeutic drug monitoring would be particularly helpful to guide dose adjustments in patients with elevated uracil and renal impairment.

Screening for dihydropyrimidine dehydrogenase deficiency by measuring uracilemia in chronic kidney disease patients is associated with a high rate of false positives

BACKGROUND

Pretherapeutic screening for dihydropyrimidine dehydrogenase (DPD) deficiency based on the measurement of plasma uracil ([U]) is recommended prior to the administration of fluoropyrimidine-based chemotherapy. Cancer patients frequently have impaired kidney function, but the extent to which kidney function decline impacts [U] levels has not been comprehensively investigated.

METHODS

We assessed the relationship between DPD phenotypes and estimated glomerular filtration rate (eGFR) in 1751 patients who benefited on the same day from a screening for DPD deficiency by measuring [U] and [UH₂]:[U], and an evaluation of eGFR. The impact of a kidney function decline on [U] levels and [UH₂]:[U] ratio was evaluated.

RESULTS

We observed that [U] was negatively correlated with eGFR, indicating that [U] levels increase as eGFR declines. For each ml/min of eGFR decrease, [U] value increased in average by 0.035 ng/ml. Using the KDIGO classification of chronic kidney disease (CKD), we observed that [U] values >16 ng/ml (DPD deficiency) were measured in 3.6 % and 4.4 % of stage 1 and 2 CKD (normal-high eGFR, >60 ml/min/1.73m²) patients, but in 6.7 % of stage 3A CKD patients (45 to 59 ml/min/1.73m²), 25% of stage 3B CKD patients (30 to 44 ml/min/1.73m²), 22.7% of stage 4 CKD patients (15 to 29 ml/min/1.73m² and 26.7% of stage 5 CKD patients (<15 ml/min/1.73m²). [UH2]:[U] ratios were not impacted by kidney function.

CONCLUSION

DPD phenotyping based on the measurement of plasma [U] in patients with decreased eGFR is associated with an exceedingly high rate of false positives when kidney function decline reaches 45 ml/minute/1.73m² of eGFR or lower. In this population, an alternative strategy that remain to be evaluated would be to measure the [UH₂]:[U] ratio in addition to [U].

Current diagnostic and clinical issues of screening for dihydropyrimidine dehydrogenase deficiency

Fluoropyrimidine drugs (FP) are the backbone of many chemotherapy protocols for treating solid tumours. The rate-limiting step of fluoropyrimidine catabolism is dihydropyrimidine dehydrogenase (DPD), and deficiency in DPD activity can result in severe and even fatal toxicity. In this review, we survey the evidence-based pharmacogenetics and therapeutic recommendations regarding DPYD (the gene encoding DPD) genotyping and DPD phenotyping to prevent toxicity and optimize dosing adaptation before FP administration. The French experience of mandatory DPD-deficiency screening prior to initiating FP is discussed.

Assay performance and stability of uracil and dihydrouracil in clinical practice

PURPOSE

Measurement of endogenous uracil (U) is increasingly being used as a dose-individualization method in the treatment of cancer patients with fluoropyrimidines. However, instability at room temperature (RT) and improper sample handling may cause falsely increased U levels. Therefore we aimed to study the stability of U and dihydrouracil (DHU) to ensure proper handling conditions.

METHODS

Stability of U and DHU in whole blood, serum, and plasma at RT (up to 24 h) and long-term stability (≥ 7 days) at - 20 °C were studied in samples from 6 healthy individuals. U and DHU levels of patients were compared using standard serum tubes (SSTs) and rapid serum tubes (RSTs). The performance of our validated UPLC-MS/MS assay was assessed over a period of 7 months.

RESULTS

U and DHU levels significantly increased at RT in whole blood and serum after blood sampling with increases of 12.7 and 47.6% after 2 h, respectively. A significant difference (p = 0.0036) in U and DHU levels in serum was found between SSTs and RSTs. U and DHU were stable at - 20 °C at least 2 months in serum and 3 weeks in plasma. Assay performance assessment fulfilled the acceptance criteria for system suitability, calibration standards, and quality controls.

CONCLUSION

A maximum of 1 h at RT between sampling and processing is recommended to ensure reliable U and DHU results. Assay performance tests showed that our UPLC-MS/MS method was robust and reliable. Additionally, we provided a guideline for proper sample handling, processing and reliable quantification of U and DHU.

Implementation and clinical benefit of DPYD genotyping in a Danish cancer population

BACKGROUND

In 2020, the European Medicines Agency recommended testing patients for dihydropyrimidine dehydrogenase (DPD) deficiency before systemic treatment with fluoropyrimidines (FP). DPD activity testing identifies patients at elevated risk of severe FP-related toxicity (FP-TOX). The two most used methods for DPD testing are DPYD genotyping and DPD phenotyping (plasma uracil concentration). The primary objective of this study was to compare the overall frequency of overall grade ≥3 FP-TOX before and after the implementation of DPYD genotyping.

PATIENTS AND METHODS

Two hundred thirty Danish, primarily gastrointestinal cancer patients, were DPYD-genotyped before their first dose of FP, and blood was sampled for post hoc assessment of P-uracil. The initial dose was reduced for variant carriers. Grade ≥3 FP-TOX was registered after the first three treatment cycles of FP. The frequency of toxicity was compared to a historical cohort of 492 patients with post hoc determined DPYD genotype from a biobank.

RESULTS

The frequency of overall grade ≥3 FP-TOX was 27% in the DPYD genotype-guided group compared to 24% in the historical cohort. In DPYD variant carriers, DPYD genotyping reduced the frequency of FP-related hospitalization from 19% to 0%. In the control group, 4.8% of DPYD variant carriers died due to FP-TOX compared to 0% in the group receiving DPYD genotype-guided dosing of FP. In the intervention group, wild-type patients with uracil ≥16 ng/ml had a higher frequency of FP-TOX than wild-type patients with uracil <16 ng/ml (55% versus 28%).

CONCLUSIONS

We found no population-level benefit of DPYD genotyping when comparing the risk of grade ≥3 FP-TOX before and after clinical implementation. We observed no deaths or FP-related hospitalizations in patients whose FP treatment was guided by a variant DPYD genotype. The use of DPD phenotyping may add valuable information in DPYD wild-type patients.

Renal impairment and abnormal liver function tests in pre-therapeutic phenotype-based DPD deficiency screening using uracilemia: a comprehensive population-based study in 1138 patients

Background

Dihydropyrimidine dehydrogenase (DPD) deficiency screening is a pre-therapeutic standard to prevent severe fluoropyrimidine-related toxicity. Although several screening methods exist, the accuracy of their results remains debatable. In France, the uracilemia measurement is considered the standard in DPD deficiency screening. The objective of this study was to describe the hyperuracilemia (⩾16 ng/mL) rate and investigate the influence of hepatic and renal impairment in uracilemia measurements since the guidelines were implemented.

Patients and methods

Using a cohort of 1138 patients screened between 18 October 2018 and 18 October 2021, basic demographic characteristics, date of blood sampling, and potential biological confounders including liver function tests [aspartate aminotransaminase (AST), alanine aminotransaminase (ALT), gamma-glutamyl transferase (γGT), alkaline phosphatase (ALP), and bilirubin] and estimated glomerular filtration rate (eGFR) were collected. The second same-patient uracilemia analysis was also performed. Temporal change was graphically represented while potential confounders were stratified to show linearity when suspected.

Results

Hyperuracilemia was diagnosed in 12.7% (n = 150) samples with 6.7%, 5.4%, 0.5%, and 0.08% between 16 and 20 ng/mL, 20 and 50 ng/mL, 50 and 150 ng/mL, and >150 ng/mL, respectively. The median uracilemia concentration was 9.4 ng/mL (range: 1.2 and 172.3 ng/mL) and the monthly hyperuracilemia rate decreased steadily from >30% to around 9%. Older age, normalized AST, γGT, ALP results, bilirubin levels, and decreased eGFR were linearly associated with higher plasma uracil concentrations (all p < 0.001). In the adjusted multivariate linear model, AST, eGFR, and ALP remained associated with uracilemia (p < 0.05). When measured twice in 39 patients, the median uracilemia rate of change was -2.5%, which subsequently changed the diagnosis in nine patients (23.1%).

Conclusions

Better respect of pre-analytical conditions may explain the steady decrease in monthly hyperuracilemia rates over the 3 years. Elevated AST, ALP levels, and reduced eGFR could induce a false increase in uracilemia and second uracilemia measurements modified the first DPD deficiency diagnosis in almost 25% of the patients.

Renal impairment and DPD testing: watch out for false-positive results!

Measuring uracil (U) levels in plasma is a convenient surrogate to establish DPD status in patients scheduled with 5-fluorouracil (5-FU) or capecitabine. To what extent renal impairment could impact on U levels and thus be a confounding factor is a rising concern. Here, we report the case of a cancer patient with severe renal impairment scheduled for 5-FU-based regimen. Determination of his DPD status was complicated because of his condition and the influence of intermittent hemodialysis when monitoring U levels. The patient was initially identified as markedly DPD-deficient upon U measurement (i.e., U = 40 ng/ml), but further monitoring between and immediately after dialysis showed mild deficiency only (i.e., U = 34 and U = 19 ng/ml, respectively). Despite this discrepancy, starting dose of 5-FU was cut by 50% upon treatment initiation. Tolerance was good and 5-FU dosing was next shifted to 25% reduction, then further shifted to normal dosing at the 5^th course, with still no sign for drug-related toxicities. Further DPYD genotyping showed none of the 4 allelic variants usually associated with loss of DPD activity. Of note, the excellent tolerance upon standard dosing strongly suggests that this patient was actually not DPD-deficient, despite U values always above normal concentrations. This case report highlights how critical is the information regarding the renal function of patients with cancer when phenotyping DPD using U plasma as a surrogate, and that U accumulation in patients with such condition is likely to yield false-positive results.